This application is based on an application No. H11-237563 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a stream demultiplexing device for demultiplexing a transport stream superimposed on a broadcast wave into a plurality of elementary streams.
2. Description of the Prior Art
In the semiconductor market, a one-chip LSI for use in a set-top box is expected to grow in demand in the years ahead.
A set-top box is a device that receives a broadcast wave via satellite, ground wave, or cable, demultiplexes a transport stream superimposed on the broadcast wave into elementary streams, and decodes the elementary streams to produce output such as video and audio signals. Set-top boxes, when used with television sets or personal computers in ordinary households, allow users to enjoy a variety of information services provided via satellite, ground wave, and cable. For this reason, set-top boxes are likely to become necessities in the coming multimedia age.
A one-chip LSI for use in a set-top box (one-chip STB-LSI) is an integrated circuit that is made up of a transport decoder (e.g. Matsushita MN88461A) for demultiplexing a transport stream transmitted on a broadcast wave into MPEG elementary streams such as video and audio streams, an AV decoder (e.g. Matsushita MN67750 and MN67751) for decoding the demultiplexed video and audio streams, and a microprocessor (e.g. Matsushita MN103002A) for controlling input/output in the set-top box. Such a one-chip STB-LSI, when equipped in a set-top box, realizes all basic operations required of the set-top box, with it being possible to simplify the wiring in the set-top box and bring down the cost of manufacturing of the set-top box.
In this one-chip STB-LSI, the demultiplexing of the transport stream is the upstream operation. Hence the transport decoder is required to perform the demultiplexing at high speed. Since the transport stream carries an enormous number of transport packets in which a plurality of elementary streams are time-division multiplexed, transfer and latch of transport packets are frequently performed inside the transport decoder.
Such frequent transfer and latch of transport packets are done by vast numbers of flip-flops mounted on the one-chip STB-LSI. In each flip-flop, as long as a clock input is provided with a synchronous clock signal for synchronous control, a logic element repeats charge/discharge in accordance with the synchronous clock signal. Given that a one-chip LSI is generally constructed of an MOS (Metal Oxide Semiconductor) LSI, the above repetition of charge/discharge with the synchronous clock signal in the logic element of each flip-flop causes massive power consumption in the one-chip STB-LSI.
The present invention aims to provide a stream demultiplexing device for demultiplexing a transport stream at high speed with reduced power consumption.
The above object can be achieved by a stream demultiplexing device for demultiplexing a plurality of elementary streams from a transport stream in which the plurality of elementary streams are time-division multiplexed, the transport stream including a plurality of transport packets that each contain a payload carrying a data portion taken from one of the plurality of elementary streams, the stream demultiplexing device including: a storing unit having a plurality of storage areas each for storing a different one of the plurality of elementary streams; a signal generating unit for generating a synchronous clock signal; a payload processing unit for obtaining, after the stream demultiplexing device starts receiving a present transport packet, a payload of the present transport packet and transferring a data portion in the obtained payload to a storage area in the storing unit that corresponds to an elementary stream from which the data portion was taken, in accordance with the synchronous clock signal supplied from the signal generating unit; and a first clock supply controlling unit for stopping the supply of the synchronous clock signal to the payload processing unit when the payload processing unit finishes transferring the data portion, and resuming the supply a predetermined time period after the stream demultiplexing device starts receiving a transport packet that follows the present transport packet.
Suppose time required for inputting and processing a payload is denoted by p, and time required for inputting a transport packet is denoted by L. Then a time period of supplying the synchronous clock signal to the payload processing unit for one transport packet is reduced to p/L according to the above construction. With such a curtailment of synchronous clock signal supply to the payload processing unit, power consumption of the payload processing unit is reduced even if it is equipped with an enormous number of flip-flops. As a result, power consumption in the stream demultiplexing device is reduced, with it being possible to realize a lower-power one-chip STB-LSI.
Here, the plurality of elementary streams may include a video stream generated by encoding a video signal and an audio stream generated by encoding an audio signal, wherein after the video stream and the audio stream have been reconstructed in the storing unit from the data portions transferred by the payload processing unit, the video stream and the audio stream are decoded using a dedicated decoder outside the stream demultiplexing device.
Even when the stream demultiplexing device is equipped with the payload processing unit of high functionality and large circuitry to efficiently demultiplex the transport stream into the plurality of elementary streams such as video and audio streams, power consumption of the payload processing unit is reduced due to the above curtailment of synchronous clock signal supply to the payload processing unit. Therefore, power consumption in the stream demultiplexing device is reduced.
Here, the stream demultiplexing device may further include the header processing unit that includes: a reading unit for reading an identifier from the header of the present transport packet; a holding unit for holding a predetermined tuning identifier in advance; and a judging unit for judging whether the identifier read by the reading unit matches the tuning identifier held in the holding unit, and notifying the read identifier to the payload processing unit if the read identifier matches the tuning identifier, wherein the first clock supply controlling unit includes a stopping unit for stopping the supply of the synchronous clock signal to the payload processing unit when the judging unit in the header processing unit judges that the read identifier does not match the tuning identifier, and wherein the first clock supply controlling unit resumes the supply which was stopped by the stopping unit, the predetermined time period after the stream demultiplexing device starts receiving the following transport packet.
When video and audio streams that provide numerous different broadcast programs are multiplexed in the transport stream, the synchronous clock signal is not supplied to the payload processing unit if an inputted transport packet does not meet a predetermined tuning pattern. In so doing, power consumption in the stream demultiplexing device is further reduced.
Here, at least one of the plurality of elementary streams may have been encrypted before being multiplexed in the transport stream, wherein the header processing unit includes: a reading unit for reading an identifier from the header of the present transport packet; a holding unit for prestoring an identifier identifying the encrypted elementary stream; and a judging unit for judging whether the identifier read by the reading unit matches the identifier in the holding unit, the stream demultiplexing device further including: a decrypting unit for obtaining, if the judging unit in the header processing unit judges that the read identifier matches the identifier in the holding unit, the payload of the present transport packet and decrypting the data portion in the obtained payload, in accordance with the synchronous clock signal supplied from the signal generating unit; and a third clock supply controlling unit for stopping the supply of the synchronous clock signal to the decrypting unit when the decrypting unit finishes decrypting the data portion, and resuming the supply when the judging unit judges that an identifier in a header of the following transport packet matches the identifier in the holding unit, wherein if the judging unit judges that the read identifier matches the identifier in the holding unit, the payload processing unit obtains the decrypted data portion generated by the decrypting unit and transfers the decrypted data portion to the storing unit, in accordance with the synchronous clock signal.
When one of the plurality of elementary streams multiplexed in the transport stream has been encrypted, the synchronous clock signal is supplied to the decrypting unit if the inputted transport packet contains a data portion which needs to be decrypted. Otherwise, the synchronous clock signal is not supplied to the decrypting unit. By such a curtailment of synchronous clock signal supply to the decrypting unit, power consumption of the decrypting unit is reduced even if the decrypting unit has large circuitry, so that power consumption in the stream demultiplexing device is reduced.
Here, the payload of each transport packet in the transport stream may further carry an identifier identifying the elementary stream from which the data portion in the payload was taken, wherein the payload processing unit includes: a formatting unit for extracting the data portion from the payload of the present transport packet, after the stream demultiplexing device starts receiving the present transport packet; and a filtering unit holding a plurality of identifiers in advance, for transferring the extracted data portion to the storing unit if an identifier in the payload of the present transport packet matches any of the plurality of identifiers, and rejecting the data portion if the identifier in the payload does not match any of the plurality of identifiers, wherein the first clock supply controlling unit includes a stopping unit for stopping the supply of the synchronous clock signal to the payload processing unit when the filtering unit rejects the data portion, and wherein the first clock supply controlling unit resumes the supply which was stopped by the stopping unit, the predetermined time period after the stream demultiplexing device starts receiving the following transport packet.
When implementing the broadcast program search service in the payload processing unit that has become a popular feature of a set-top box in recent years, the synchronous clock signal supply to the payload processing unit is discontinued if the inputted transport packet does not meet any search pattern. This further reduces power consumption in the stream demultiplexing device.
Here, the payload processing unit may include a plurality of processing subunits each for processing a payload in accordance with the synchronous clock signal supplied from the signal generating unit, wherein the header processing unit includes a selecting unit for selecting one of the plurality of processing subunits which is to process the payload of the present transport packet, based on an identifier in the header of the present transport packet, and wherein the first clock supply controlling unit stops the supply of the synchronous clock signal to the other processing subunits when the selecting unit selects the processing subunit, and resumes the supply when the selected processing subunit finishes transferring the data portion and the stream demultiplexing device starts receiving the following transport packet.
When the stream demultiplexing device is equipped with a plurality of payload processing subunits for separately processing the plurality of elementary streams, the synchronous clock signal is supplied only to a payload processing subunit that is to process the inputted transport packet, and is not supplied to the other payload processing subunits. By such prohibiting the synchronous clock signal supply to the payload processing subunits which are being idle, power consumption of the plurality of payload processing subunits is reduced, with it being possible to reduce power consumption in the stream demultiplexing device.